because every time Cas play, he is their best player and he usually picks up a point or 2. At Saints & Wigan, the points are usually split amongst 4 or 5 players so no one individual can get enough to win it.
No one would choose McShane over Roby, or even Powell, based on form this season - but neither of those 2 are in the Top 5 because they have shared the points with their team mates.